There are known hydrogenated products of aromatic vinyl polymers, which are obtained by hydrogenation of the aromatic rings of the aromatic vinyl polymers such as polystyrene, and hydrogenated products of aromatic vinyl copolymers, which are obtained by hydrogenation of the copolymers produced from aromatic vinyl compounds and vinyl compounds. For example, as disclosed in Japanese Patent Laid-Open No. 1-317728, hydrogenated products of polymers such as a hydrogenated polystyrene resin, a hydrogenated product of styrene-butadiene-styrene block copolymer, and the like (hereinafter, referred to as hydrogenated polystyrene resin, as the case may be) are small in refractive index, so that they are suitable for use as the molding material of optical disks.
It has been proposed that in order to increase the recording density of an optical disk, two or more sheets of thin substrate are bonded together, and a short wavelength laser is used to record and play back the information. A substrate for such a high recording density optical disk is required to have a high mechanical strength and an excellent light transmittance. However, in the above described hydrogenated products of polymers, the mechanical strength, in particular, bending strength is low, and furthermore, the birefringence and light transmittance have been demanded to be improved.
Specifically, the hydrogenated polystyrene resin is insufficient in mechanical strength as compared to poly(methyl methacrylate) resin (PMMA) used as molding material for optical parts, and hence it has been difficult to use the hydrogenated polystyrene as the material for the optical parts other than optical disks and miniature lenses, in particular, for plate molded objects. As a plate molded object made of PMMA, there can be cited an optical waveguide for back light in a liquid crystal display element (LCD). In these years, large, thin, and light optical waveguides have been demanded, and accordingly the heat resistance and the like are required for the materials concerned.
The styrene-ethylene-butylene-styrene block copolymer (SEBS), which is a hydrogenated product of the styrene-butadiene-styrene block copolymer (SBS), has the heat resistance, weather resistance, and oil resistance, improved by hydrogenation as compared to those of SBS, and accordingly its applications to a variety of uses have been studied. Lately, SEBS is used as sheets for use in civil engineering and construction, and a study has started on its use in the medical field. However, SEBS is not high in elastic modulus, and hence it is restricted to the use as relatively thick sheets. In addition, SEBS is not transparent, and accordingly its use in the optical field has been difficult.
International Publication WO98/55886 describes the hydrogenated product of an aromatic vinyl polymer, in which the aromatic ring hydrogenation rate is 97% or more, the weight average molecular weight is from 100,000 to 300,000, and the molecular weight distribution is 2.0 or less, and which can be used as polarization film and phase difference film. However, this polymer has been invented as a material suitable for use in formation of plastic lenses excellent in low birefringence, and there are disclosed only the possibilities for use in such special optical films as described above. Recently, there is a high demand for polymer films in the fields of information, medical services, and food products; there is a demand for films and sheets which are excellent in transparency, heat resistance, mechanical strength, and the like, and can be used in a variety of uses.
Additionally, SEBS is not high in elastic modulus, so that it has been difficult to form vessels of SBBS by injection molding or blow molding SBBS and to use them. On the other hand, Japanese Patent Laid-Open No. 6-199950 discloses that a molded object made of a vinylated cyclic hydrocarbon polymer or a hydrogenated product thereof is moderate in transparency, and is excellent in chemical resistance, low elution, electric insulation, and the like, so that it is useful for medical instruments and materials, electric insulation materials, and materials for processing electric parts, and can be used for chemical substance containers and food containers.
However, the containers made of the above described polymer sufficiently meet the usage as containers standing still and simply holding things; but when they are used in such a way that they suffer bending stress or light impact, in particular, in a manner of being repeatedly used, in a manner of being carried about, or in a manner of being superposed and stacked, there comes about a drawback that cracking and fracturing occur owing to their insufficient mechanical strength. Recently, in the fields of medical care and food industry, there are demands for containers excellent in transparency, heat resistance, mechanical strength, and the like.
Furthermore, in parallel with the demand for downsizing the apparatus for reading signals from a magneto-optical disk which is a recording medium of a computer, a pickup lens is demanded to be downsized and made uneven in thickness. Such an optical lens tends to exhibit birefringence when downsized and made uneven in thickness. Accordingly, there is expected the advent of an optical lens formed by use of a material further low in birefringence.
As for laser printers, in parallel with the demand for use of large-sized sheets of paper, there is a demand for an upsized, thin, light single-layer lens as the fθ lens, instead of a conventional thick stacked plano-convex lens. On the other hand, there is also a demand for downsizing the laser printers, and accordingly there has come out a demand for a meniscus-convex lens downsized and made uneven in thickness. As a response to such a variety of needs, it is necessary to bring about the advent of a lens, transparent and low in birefringence, provided with such a mechanical strength as can actualize the molding of demanded shapes.